A fuel vapor treatment apparatus directly purges fuel vapor, which is produced in a fuel tank, into an intake passage of an internal combustion engine. Alternatively, a fuel vapor treatment apparatus temporarily adsorbs fuel vapor to an adsorbent of a canister and then purges the adsorbed fuel vapor into the intake passage. In the fuel vapor treatment apparatus according to JP-A-H5-18326 or JP-A-H6-101534, a fuel vapor concentration in a mixture to be purged into the intake passage is measured as a fuel vapor state prior to the purge. Concretely, the flow rate or density of the mixture is detected in a purge passage through which the mixture is purged into the intake passage. In addition, the flow rate or density of air is detected in an atmospheric passage, which opens to the atmosphere.
The fuel vapor concentration is measured in accordance with the ratio between the detection results of the purge passage and the atmospheric passage. In the above structure, negative pressure in the intake passage is applied to each of the passages, and the mixture or air flows through the corresponding passage, whereby the flow rate or density is detected. When a pulsation occurs in negative pressure through the intake passage, the flow rate or density fluctuates, and the measurement accuracy of the fuel vapor concentration decreases. Besides, when negative pressure of the intake passage is small, the flow rate of the mixture or air in the corresponding passage decreases. Consequently, the detection of the flow rate or density becomes difficult.
Fuel vapor produced in the fuel tank may flow into a measurement passage separate from the purge passage, when the measurement passage is blocked from the intake passage. Here, the fuel vapor concentration is measured in such a way that a physical quantity such as pressure or flow rate correlating to the fuel vapor concentration is detected in the measurement passage. Accordingly, fuel vapor or air flows through the measurement passage irrespective of the fluctuation of negative pressure of the intake passage, and the fuel vapor concentration may be precisely measured.
Fuel vapor is purged into the intake passage on the basis of the measured fuel-vapor concentration. A quantity of fuel injected from a fuel injection valve is set in accordance with a quantity of fuel vapor, which is to be purged. In this regard, when a malfunction occurs in any of components for measuring the fuel vapor state, the measurement cannot be accurately performed. As a result, the quantity of fuel injection cannot be appropriately set, and consequently, an actual air/fuel ratio may deviate from a target air/fuel ratio.